Equilibrium melting curves of silver using high temperature calorimeters

Abstract

A high-temperature calorimeter has been developed for studying the equilibrium melting behaviour of substances used for realizing the thermometric fixed points. Conventional cells of either 5 cm or 2.5 cm diameter can be accommodated. Using the calorimeter, the equilibrium melting curves of three high-purity silver samples were measured. The samples have narrow melting ranges: two of 1.5 mK and one of about 0.5 mK from 10 % liquid sample fraction to the last measured point near 70 % liquid, yet their liquidus points differ by nearly 5 mK. It was found that 1/ plots cannot be used to determine the deviation of the melting point of real silver samples from that of ideally pure silver.

Realization of the Indium Fixed Point by an Adiabatic Technique

“…Nevertheless, several experiments under adiabatic conditions have already been reported in the field of metal fixed points. These generally consisted of keeping the surroundings as isothermal as possible, with respect to the fixed-point cell, by means of heaters and shields [3,4,10,12,13].…”

“…4). Run-off was followed by melt-off [3], showing the difficulty in estimating the liquidus point and the melting time using the continuous heat-flux method.…”

“…In particular, the observed temperature is quite different from that of the solid/liquid interface at the end of melting ("run-off" point) and during freezing when slowly recovering from the supercool. The first consequence is that solid/liquid fractions along the freezing curves cannot be determined accurately [3]. Secondly, the lack of thermal models to account for the real behavior of fixed points during the entire phase change results in difficulty in distinguishing thermal and impurity effects [4], especially following recalescence.…”

“…Compared to the situation described above, the thermometer should ideally indicate the temperature of the solid/liquid interface no matter what the direction of the phase transition, solid-to-liquid or liquid-to-solid. During the CCT workshop held in Cavtat in 2004 [8], Hill discussed selected investigations of metal fixed points realized both under heat flux and adiabatic conditions [3,4,9,10]. In particular, he concluded his talk with the following question "Do temperatures realized by adiabatic and continuous heat-flux techniques differ in principle?…”

Heat flow metric policy for dynamic monitoring of fixed-point cells

“…The main concept, keystone of such a device, hinges on a generic cell inserted into another one arranged as an isothermal structure: to this end, the isothermal condition is ensured by way of the same material in its own solid-liquid changing state than that experimented with. Considering as verified the mandatory metrological measurement provisions [2], such a cell should allow better performance than the same unit that would be directly arranged within a standard controlled furnace as its medium environment. A real and effective thermal shield is allowed thanks to both transitions occurring simultaneously, with the straightforward provision met as the load experimented with in the inner cell is far lower then that inserted within its encompassing guard.…”